Persistent mode superconductive orthogonal gradient cancelling coils

ABSTRACT

A superconductive magnet is disclosed together with superconductive gradient cancelling coils for homogenizing the magnetic field produced by the magnet. A gyromagnetic spectrometer is also disclosed which employs the magnetic field produced by the superconductive magnet. The gradient cancelling coils include a plurality of turns of a superconductor arranged adjacent the region of the magnetic field to be corrected, such magnetic field having certain residual magnetic field gradients therein to be cancelled. Means are provided for energizing the gradient cancelling coils with electrical currents in such a configuration as to define orthogonal superconductive gradient cancelling coils. A superconductive connection is formed across the ends of the coil structure for closing the superconductive circuit across the ends of the coil to form a closed superconductive circuit, whereby the superconductive gradient cancelling coils may be operated in a persistent mode for enhanced stability and reduced power consumption. In one embodiment, a DC transformer is provided which has a primary winding coupled to the main magnetic field of the magnet and a secondary winding which serves to energize the gradient cancelling coils. In this manner, the gradient cancelling field components change their intensity in proportion to changes in intensity of the main magnetic field to maintain a corrected field.

United States Patent Rev. of. Sci. lnstr. v.31, No.4, Apr. 1960 pp. 369-373. (Autler) Journal of Applied Physics v.34, No. 1 1, Nov., 1963, pp.3175 3178. -(Marshall) RANSMITTER RECEIVER RECORDER PrimaryExaminer-Rudolph V. Rolinec Assistant Examiner-Michael J. LynchAttorney-Leon F. Herbert ABSTRACT: A superconductive magnet is disclosedtogether with superconductive gradient cancelling coils for homogenizingthe magnetic field produced by the magnet. A gyromagnetic spectrometeris also disclosed which employs the magnetic field produced by thesuperconductive magnet. The gradient cancelling coils include aplurality of turns of a superconductor arranged adjacent the region ofthe magnetic field to be corrected, such magnetic field having certainresidual magnetic field gradients therein to be cancelled.

Means are provided for energizing the gradient cancelling coils withelectrical currents in such a configuration as to define orthogonalsuperconductive gradient cancelling coils. A superconductive connectionis formed across the ends of the coil structure for closing thesuperconductive circuit across the ends of the coil to form a closedsuperconductive circuit, whereby the superconductive gradient cancellingcoils may be operated in a persistent mode for enhanced stability andreduced power consumption. In one embodiment, a DC transformer isprovided which has a primary winding coupled to the main magnetic fieldof the magnet and a secondary winding which serves to energize thegradient cancelling coils. in this manner, the gradient cancelling fieldcomponents change their intensity in proportion to changes in intensityof the main magnetic field to maintain a corrected field.

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INVENTOR HARRY HI SHEET 1 BF 3 ER JR. W

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V PATENTED m 4m 3; 577,067

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2 AXIS Ho CENTER 45 2cl ZAXIS RECORDER SEA - POWE GENERATOR RANSHITTERRECEIVER P LY INVENTOR HAR Y E. ISIA'VER JR.

ORNEY PATENTED MAY 4 l97l SHEET 3 UF 3 PERSlSTEN'l Mom; SUPERCONDUCTIVEORTHOGONAL' GRADIENT CANCELL'ING COILS Heretofore, superconductivegradient cancelling coils have been used with superconductive solenoidsfor improving the uniformity of the magnetic field. Such a system isdescribed and claimed in copending US. application Ser. No. 483,402

filed Aug. 30, 1965 and assigned to the same assignee as the presentinvention. In this prior magnet system, certain resistive networks wereconnected across the terminals of the various ones of the gradient coilsfor determining the amplitudes of their respective energizing currents.The problem with this prior arrangement was that it included no meansfor operating the various field correction coil sets in the persistentmode, i.e., a completely superconductive closed circuit loop such thatcurrents, once initiated, continue to flow undiminished for anindefinite period of time. Thus, the prior field corrective coils couldnot be disconnected from their power supply and power continued to beconsumed by the coil sets. Furthermore, the field corrections weresubject to minute superconductive loops for cancelling residual gradientcomponents of the main solenoid. Such a system is described in anarticle entitled Application of the Garrett Method to Calculation ofCoil Geometries for Generating Homogeneous Magnetic Fields inSuperconducting Solenoids" appearing in Journal of Applied Physics, vol.34, No. l l of Nov. 1963. However, in this prior system the windingsegments were not energized in an orthogonal manner with respect to eachother such that a change of the current in one winding segment which wasmade to cancel a certain residual gradient produced a change in thetotal magnetic field intensity at the center of the solenoid and alsoproduced a magnetic field gradient which interfered with a previouslyoptimized settingof another gradient cancelling winding segment.

In the present invention the coil sets are orthogonal with respect toeach other and to the main field of the solenoid. at its center. Meansare provided for energizing the orthogonal field corrective coil setsand for operating the coil sets in the persistent mode, whereby in theiroperating persistent mode they draw no power from an external powersupply, and whereby the stability of their correction is enhanced. Inanother embodiment of the present invention, the persistent modeorthogonal field correction coils are coupled to the magnet producingthe field which is being corrected in such a manner that the persistentmode currents in the field corrective coils vary in amplitude withchanges in the intensity-of the field being corrected. In this mannerthe main magnetic field may be scanned in intensity and the fieldcorrective coils will change their field corrective components tomaintain the proper correction, as previously established.

As used herein orthogonal coil" means that the coil as energizedproduces a gradient cancelling field corrective component which does notsubstantially change the total magnetic field intensity over the regionof the main field that is being corrected nor does it produce .asubstantial gradient component that will interfere with the previouslyoptimized settings of other gradient cancelling coils arranged forcorrecting the field. Orthogonal gradient cancelling coil configura-.tions may be obtained by shaping the current paths of the variousseparately energizable coils in certain predetermined patterns as taughtin copending U.S. application Ser. No. 348,442 filed Mar. 2, 1964 nowUS. Pat. No. 3,287,630 and as taught in Ser. No. 441,829 now US. Pat.N0. 3,469,180 filed Mar. 22, 1965 or by adopting certain coil patternsand superimposing different combinations of current through the variouscoils as taught in copending US. application Ser. No. 442,000 now US.Pat. No. 3,488,561 filed Mar. 23, 1965 all assigned'to the same assigneeas the present invention.

The principal object of the present invention is the provision of apersistent mode superconductive orthogonal magnetic field gradientcancelling coil.

One feature of the present invention is the provision of an orthogonalsuperconductive gradient cancelling coil having means for forming aclosed superconductive circuit loop, whereby the gradient cancellingcoil may be operated in the persistent mode to enhance the stability ofits field corrective component and whereby its operating powerconsumption is negligible.

Another feature of the present invention is the same as the precedingfeature wherein the superconductive circuit loop is closed through aswitch means, whereby the coil may be energized to a certain currentamplitude from an external power supply and then switched into thepersistent mode to sustain the predetermined field correction.

Another feature of the present invention is the same as the firstfeature wherein the superconductive gradient cancelling coil is coupledto the magnet producing the field to be corrected either by means of aDC field transformer or by being closely spaced to the field regionbeing corrected, such that the current in the gradient cancelling coilautomatically changes its magnitude in accordance with changes in theintensity of the main field of the magnet to maintain the previouslyestablished field correction.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the superconductivegradient'cancelling coils are formed by segments of the windings of thesuperconductive solenoid which produces the main field of the magnet.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic circuit diagram, partly in block diagram form of asuperconductive magnet system employing features of the presentinvention;

FIGS. 21-d are plots of magnetic field intensity H versus distance, d,along the Z-axis of the magnet of FIG. 1 and depicting the effect of theZ-axis gradient cancelling coils of FIG.

FIG. 3 is a schematic longitudinal sectional view, partly in blockdiagram form, of a gyromagnetic resonance spectrometer employing themagnet system of FIG. 1;

FIG. 4 is an enlarged side elevational view of a set of orthogonalgradient cancelling coils of FIG. 3 as delineated by line 4-4;

FIG. 5 is a schematic circuit diagram for energizing the gradientcancelling coils of FIG. 4', and

FIG. 6 is a schematic plan view of the DC current transformer portion ofFIG. 1 as delineated by line 6-6.

Referring now to FIG. 1 there is shown in schematic form a circuitdiagram-of a superconductive magnet system employ ing features of thepresent invention. A superconductive wind ing, which may comprise, forexample, 120,000 feet of copper jacketed NbZr wire is wound into asolenoid 1, 12 inches long and 1.5 inches in inside diameter. Thesolenoid l is preferably. of the type having additional series-connectedwindings at the ends to bring up the intensity of the field near theends. The solenoid is tapped at a number of places along the length ofthe solenoidwinding such as, for example, at 12,000 foot intervals ofthe winding. These taps are brought out of a cryostat 2, in which thesolenoid l is immersed, via copper leads 3 to a bank of forward andbackward conducting diodes 4 with one backward and one forwardconducting diode connected across each tapped segment of the solenoidwinding 1. The provision of these diodes 4 protects the solenoid l' andpower supply in case the solenoid quenches, and. forms the subjectmatter of and is claimed in copending US. application Ser.

No. 543,666 now U.S. Pat. No. 3,474,294 filed Apr. 19, I966,

and assigned to the same assignee as the present'invention.

A main power supply, 5, which is of the current controlled or regulatedtype, supplies the energizing current of 0 to 25 amps to the solenoid 1via leads 6. Three additional power supplies .7, 8, and 9, which are ofthe current controlled type, deliver 1':

1 amp of current at 3 volts to their respective loads. The end powersupplies 7 and 9 are each connected across the end winding segments ofthe solenoid 1. Each end winding segment comprises for example I percentof the total number of windings of the solenoid I. The center powersupply 8 is connected across the central section of winding segmentswhich comprises about 80 percent of the total number of windings of thesolenoid I. The power supplies 7, 8, and 9 have a near infinite outputimpedance and each comprises an operational amplifier followed by a highfidelity audio power amplifier output stage with its capacitors removedto provide DC response. The output current of the power supplies 7 8 and9, is supplied to the two end and central windings sections, iscontrolled by the input signals applied to the power supplies 7, 8, and9 from their respective potentiometer networks 1 I, I2 and 13.

The two end power supply inputs are ganged together via a shaft 14driving the pickoff arms of a pair of potentiometers l and 16 which areconnected across the terminals of grounded centertapped battery suppliesl7 and 18. Turning the shaft I4 causes the respective pickoffs of thepotentiometers I5 and 16 to provide equal and opposite input signals tothe power supplies 7 and 9 via leads l9 and 21, respectively. In thismanner, plus current is supplied to one end segment of the solenoid andan equal minus current is supplied to the other end segment of thesolenoid. These plus and minus currents are superimposed upon the mainsolenoid current supplied from the main power supply 5 to produce alinear orthogonal gradient component superimposed upon a certainresidual linear gradient component of the main magnetic field of thesolenoid l for cancelling a certain residual gradient in a manner morefully described below. Variable resistors 22 and 23 are placed in theleads l9 and 2t for providing a fine adjustment in the relativeamplitudes of the output currents of the end power supplies 7 and 9.

Similarly, a second common shaft 24 interconnects, and thus gangs, thepotentiometer networks ll, 12 and 13 of the power supplies 7, 8, and 9to provide another orthogonal gradient control. In this instance the endsegment power supply input potentiometers 25' and 26 are connectedacross their respective grounded centertapped batteries I7 and 18 suchthat turning the shaft 24 causes their respective pickoffs to pick offinput signals of like sign, i.e., both plus or minus, such that both endpower supplies produce a like plus or minus output current. These secondinput signals are supplied to end power supplies 7 and 9 via leads 2'7and 28 with each lead including a variable resistor 29 and 31 forproviding a fine relative adjustment in the amplitudes of the inputsignals.

However, the center power supply 8 is also ganged to the shaft 24 andits input potentiometer 32 is connected across its grounded centertappedbattery 33 in such a manner that its 1 sister 35. Resistor 35 permitsadjusting the amplitude of the input signal to the center power supplyrelative to that supplied to the end power supplies 7 and 9.

The output current components produced by rotation of shaft 24 aresuperimposed upon the previously established currents in the solenoid 1produced by the main power supply 5 and end power supplies 7 and 9.Thus, rotation of shaft 24 causes the end winding segment currentcomponents to each vary alike in a like sense while the currentcomponent supplied tothe center winding section varies in the oppositesense. The result is the production of an adjustable nonlinearorthogonal axial gradient component which is superimposed upon theresidual axis nonlinear gradient component of the main magnetic field,if any, for cancelling same to render the total magnetic field of thesolenoid l more uniform.

The operation of the gradient cancelling controls 14 and 24 is moreeasily seen with regard to FIGS. 21-d. Assume that the main magneticfield H,, produced by the current from the main power'supply 5 passingthrough the solenoid l, is as shown by the solid line 41 of FIG. 2a.This field H, has a linear gradient along the axis of the solenoid l,the Z-axis. The 5 desired field would be as shown by the dotted line 42of FIG. 2a, i.e., a uniform field of constant intensity from one end ofthe solenoid I to the other; This linear gradient is cancelled in thecircuit of FIG. 1 by turning shaft 14 to produce an increase in thefield H at the low intensity end and reduce the field H, at the highintensity end, as shown by the dotted line 43 of FIG. 2b. The resultantfield is uniform as shown by the solid line 44 of FIG. 2b. Notice thatthis correction is made without changing the total magnetic fieldintensity at the center of the solenoid. This is very important becausethe field uniformity is typically monitored by, or used for,gyromagnetic resonance. If the total field intensity H, changed over theresonance sample, located at the center of the solenoid, the resonancesignal would be lost, assuming the sample were excited from aconventional fixed frequency source tuned to the Larmor frequency in thefield before the gradient correction were made. Alternatively, assumethe initial total magnetic field H had a nonlinear gradient as shown bythe solid line 45 in FIG. 2c. Turning the shaft 24 in the properdirection would introduce a corrective gradient component as shown bythe dotted lines 46 and 47 of FIG. 211. In this case, the end powersupplies 7 and 9 superimpose corrective field components 46 which, ifnot otherwise compensated, would produce a change in the total fieldintensity H at the center of the solenoid. However, this change isavoided because the center power supply 8 produces an opposite sensecurrent and field 47 which cancels, in the-center region of thesolenoid, the undesired field change produced by the end windingsegments to produce a uniform total field 48. In actual practice, theundesired residual main magnetic field gradients of the type depicted bylines 41 and 45 of FIGS. 2a and 2c are typically mixed. However, thecontrols I4 and 24 are independently adjustable or orthogonal, i.e., anoptimum setting of one control I4 does not interfere with a previouslyoptimized adjustment of the other control 24, for removing theseundesired axial gradients, and the corrections are made without changingthe total magnetic field intensity at the center of the solenoid 1.

Low pass filter networks comprising 0.50. series connected resistors 49and shunting capacitors 51, as of 2.2;tf, are connected across theoutput terminals of each of the power supplies 7, 8, and 9 to preventoscillation of the output current of the power supplies 7, 8, and 9.Likewise, the main power supply 5 includes a low pass filter, not shown,connected across its output terminals for the same reason.

Although the end and center sections of the solenoid l are employed inconnection with the separate power supplies 7, 8, and 9 for cancellingcertain residual axial gradients, still other axial and transverseresidual gradients exist which it is desirable to eliminate.Accordingly, several other orthogonal gradient cancelling coils, whichpreferably have certain prescribed orthogonal physical geometries topermit optimum independent adjustment of each without mutualinterference, are arranged adjacent the central region of the solenoidfor improving the uniformity of the field H Such a set of coils may havegeometries as indicated in FIG. 4 and may be disposed surrounding thecentral region of the main field H as shown in FIG. 3. Such a set oforthogonal coil geometries fonns the subject matter of and is claimed incopending US. application Ser No. 348,442 filed Mar. 2, 1964 now US.Pat. No. 3,287,630 and assigned to the same assignee as the presentinvention. These additional orthogonal gradient cancelling coils eachcomprise plural turns of superconductive wire and, in one embodiment ofthe present invention, they are energized by being coupled to the mainfield H, of the magnet 1 via an adjustable DC transformer 53, more fullydescribed with regard to FIG. 6. Alternatively they may be locatedinside the main solenoid ll closely surrounding the region of field tobe corrected in which case they require no DC transformer and ends ofeach coil are merely connected together by a superconductive joint.

' The transformer 53 is energized via a superconductive primary winding54 which is series connected to the superconductive main solenoidwinding 1. Superconductive secondary windings 55,- only two of which areshown for simplicity of exthus its field'corrective gradient componentis adjustable (See P10. 6), by turning its respective armature 57 onwhich the secondary winding 55 is wound. Turning the armature 57increases or decreases the magnetic fiux of the primary winding 54, asproduced in the iron core 58 of the transformer 53, which threadsthrough the respective secondary winding 55. The various armatures 57are turned via mechanical shafts, not shown, for the optimum adjustmentof the various field corrective components.

The DC transformer energized gradient cancelling coils 56 yield anoperating advantage, as compared to the end and center section windinggradient cancelling coils of the solenoid, because they are coupled tothe main field of the sole noid 1. Thus, as the main field is scanned,by changing the current supplied by the main power supply 5, therespective currents in the gradient cancelling coils and thus theircorrective field components are scanned proportionally, therebypreserving their proper preestablished field correction. In contrast,the other gradient coils, which form a part of the solenoid windings,require the end and center power supplies 7 and 9 to be readjusted tomaintain their proper corrections when the main field intensity ischanged to a new value. Also the gradient cancelling coils 56 have anadditional advantage in that they require no separate power supply.

For the case where each orthogonal coil sets 56 has its ends joinedtogether via superconductive joint and are merely placed inside thesolenoid winding 1 surrounding the central region of the main solenoidwhich is to be corrected, the currents in each of the coil sets willautomatically adjust themselves to cancel out minute residual gradientsin the main field being corrected. These corrections will beself-adjusting with scan of the main field intensity to automaticallypreserve a uniform field inside the coil sets 56. ln other words thecoil sets 56 act as secondary DC transformer windings coupled to theprimary field being corrected. Each coil set 56 orthogonally couples toits respective main field gradient component, if any, which'is beingcorrected.

' Once the main magnetic field has been corrected via the variousorthogonal gradient cancelling coils the magnet system may be switchedinto a persistent current mode, thereby preserving indefinitely thetotal corrected field If Superconductive wires 59, 61, and 62 areconnected across the ends of the two end sections and the center sectionof the main solenoid windings. Persistent switches 63, 64, and 65 areconnected in the superconductive wires 59, 61, and 62. The persistentswitches each comprise a thermally insulative dielectric member 66through which the superconductive wire 59, 61 and 62 passes. A resistiveheating element 67 is also embedded in the dielectric member 66. Heatingcurrent is supplied to each of the heating elements 67 via leads 68supplied with current from a power supply 69 as divided by and tappedoff of a voltage divider network 71. An array of ganged single poledouble throw switches 72 are connected in the leads 6 and'3interconnecting the respective power supplied 5, 7, 8, and 9 and thesolenoid 1 and in the leads 68 interconnecting the heating elements 67of the persistent switches 63, 64, and '65. A switch 60 is alsoconnected in circuit between the persistent switch power supply 69 andthe voltage divider 71.

During the time the superconductive solenoid l is being energized fromthe power supplies 5, 7, 8 and 9, the switches 60, 72 and 72 are allclosed. The heating elements'67 are thus energized thereby heating thesuperconductive wires 59, 61 and 62 to a temperature slightly abovetheir superconductive critical temperature such that they do not appearsuperconductive and thus do not bypass the current in the main superconductive solenoid winding 1. Once the corrected field condition hasbeen achieved in the main field of the solenoid magnet, as describedabove, the switch 60 is opened. The persistent switches 63, 64 and 65are thus deenergized allowing the liquid helium within the cryostat 2 tocool quickly, i.e., within 1 to 2 seconds, the superconducting bypasswires 59, 61 and 62 to their superconductive 5, 7, 8 and 9 is thenreduced to zero amplitude and as this is accomplished the magnet currentshifts from the circuit portions which included the various magnet powersupplies 5, 7, 8, and 9 to the superconductive bypass circuit loopportions. In this manner, the previously established currents in thevarious sections of the solenoid windings 1 persist indefinitely,thereby preserving the preestablished corrected field conditions withoutthe need of supplying additional power to the magnet system. Theswitches 72 and 72' are then opened to isolate the magnetfrom the powersupplies. in the persistent mode, the field has enhanced stability sinceit is now isolated from possible current fluctuations and surgesproduced by the power supplies.

Referring now to H6. 3 there is shown a gyromagnetic resonancespectrometer employing the magnet system of FIG. 1. The superconductivesolenoid l is immersed in the cryostat 2. The second gradient cancellingcoil system 56 is coaxially disposed of and outside of the main solenoidl. The cryostat 2 includes a liquid nitrogen dewar assembly 75surrounding a liquid helium dewar assembly 76 having the superconductivesolenoid 1 immersed therein. A third dewar 77 is centrally disposed ofthe liquid helium and nitrogen dewars and is open to the atmosphere. Thethird dewar 77 contains a gyromagnetic resonance sample 78 to beanalyzed. The sample 78 is contained within a glass vial 79.

The solenoid 1 produces an intense uniform unidirectional magnetic fieldH as of 54 kg., directed along the Z-axis and through the sample volume.A transmitter coil 81, which is coaxially aligned with the X-axis, isexcited with radio frequency power supplied from a transmitter 82. Thetransmitter output is at the Larmor frequency as of 220 mHz., of thegyromagnetic bodies (protons) within the sample 78 to excitegyromagnetic resonance thereof. A receiver coil 83, aligned with theX-axis, picks up the resonance signal emanating from the sample 78 andfeeds the resonance signal to a receiver 84 wherein it is amplified,detected, and fed to a recorder 85.

The magnetic field H, is scanned in intensity by superimposing a uniformDC field component H, along the Z-axis on the main field of thesolenoid 1. The scan field component is produced by a small solenoid,not shown, wound around the probe inside the main solenoid. Thescansolenoid is supplied with current from a scan generator 86. The field H,is scanned through the various resonance lines of the sample 78 toproduce a resonance output spectrum signal. The spectrum signal isrecorded in recorder 85 as a function of the scan field signal toproduce the recorded resonance spectrum of the sample 78.

Referring now to FIG. 5 there is shown an alternative gradientcancelling coil circuit embodiment of the present invention. In thisembodiment the second group of superconductive gradient cancelling coils56 are sequentially energized from one power supply and thensequentially switched into the persistent mode. More particularly, thesuperconductive gradient cancelling coils 56, as described above, areeach connected to a set of switch terminals 91' of a double polemultiple position switch 92 for sequentially connecting the variousgradient cancelling coils 56 to the power supply 8. The power supply 8is the same as the one previously described with regard to FIG. 1.Superconductive wires 93 are connected across the terminals of each ofthe superconductive gradient cancelling coils 56. Persistent switches63, as previously described, are connected in the superconductive bypasswire portion 93 of the circuit. In operation, each one of the gradientcoils 56 is sequentially connected to the power supply 8 via switch 92.The switches 60 and 72 controlling the current to the persistent switchheating elements 67, are closed.

, 7 v The selected coil 56 is then energized viathe power supply 8 tothe propercurrent to correct a certain one-of the residual gradients ofthe main field produced by the solenoid 1. When the proper fieldcorrection is obtained, the particular persistent switch 63 isdeenergized by opening its heater current switch 72. After the bypasswire 93 has cooled to its superconductive state the current from thepower supply 8 is reduced to zero and then disconnected-via switch 92.In this manner, the-established field correction, for that gradientcancelling coil, is preserved indefinitely without need of supplyingadditional power to that gradient cancelling coil 56. This procedure isrepeated for the other coils 56 until all coils 56 are properlyenergized and operating in the persistent mode.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

l. A superconductive magnet system including, means forming asuperconductive solenoid having a winding with a multitude of turns ofsuperconductor for producing a main magnetic field region within saidsolenoid having certain residual magnetic field gradient components,said winding of said 'solenoid including a pair of end winding segmentsconnected in series with an intermediate winding segment of more turnsthan each of said end winding segments, each of said winding segmentscomprising a multitude of turns of superconductor, means forming asuperconductor shunt connected in shunt across each of said windingsegments, meansincluding nonsuperconductive current carrying leadsconnected to each end of each of said end winding segments forenergizing said end winding segments from a current supply with currenthaving an amplitude which is separately variable relative to theamplitude of the current in said intermediate winding segment of saidsolenoid to produce a gradient cancelling field component, means forminga switch connected in each of said superconductor shunts which shuntsaid end winding segments, whereby each of said end winding segments maybe energized from the current supply to produce a certain gradientcancelling field component superimposed on the main field of saidsolenoid and then switched via said switch means into a persistentsuperconductive current mode to preserve the preestablished gradientcancelling field component.

2. The apparatus of claim 1 including, means forming a cryostat forcontaining said solenoid means and for cooling said solenoid to asuperconductive state, means for immersing a gyromagnetic resonancesample of matter within the gradient cancelled region of magnetic field,means for exciting and detecting gyromagnetic resonance of the sample.

3. In a superconductive magnet apparatus, means forming asuperconductive solenoid for producing a main magnetic field region tobe corrected which has certain residual magnetic field gradientcomponents therein, first and second gradient cancelling coil means forcancelling the residual magnetic field gradient components, each of saidfirst and second gradient cancelling coil means having an orthogonalcurrent path geometry relative to each other, a superconductive jointconnected across the ends of each of said first and second coil means toform first and second closed superconductive circuits, and wherein saidclosed superconductive circuits are disposed inside said solenoidsurrounding the fieldregion to be corrected, whereby each of said coilmeans acts as the secondary winding of a DC transformer and the correctcurrent amplitude will automatically flow through each of saidorthogonal coil geometries to cancel the certain residual magnetic fieldcomponents in the region to be corrected.

4. In a superconductive magnet apparatus, means forming asuperconductive solenoid for producing a main magnetic field region tobe corrected which has certain residual magnetic fieldgradientcomponents therein, first and second gradient cance ling coilmeans for cancelling the residual magnetic field gradient components,each of said first and second coil means having an orthogonal currentpath geometry relative to each other, means forming a superconductiveconnection connecting across the ends of each of said first and secondcoil means to form first and second closed superconductive circuits,means forming a direct current transformer having a primary windingenergized with current proportional to the current flowing in thewindings of said superconductive solenoid, and said superconductiveconnection which connects across the ends of each of said first andsecond coils being wound to form first and second secondary windings ofsaid direct current transformer means, whereby the gradient cancellingfield components produced by said first and second gradient cancellingcoil means have magnitudes which vary in proportion to the currentflowing in said solenoid, thereby preserving the preestablished gradientcancelling effect when the main magnetic field intensity is scanned.

5. The apparatus of claim 4 including means for varying the amount ofmagnetic coupling betweenv the primary and secondary windings of saidtransformer means for adjusting the amplitude of the gradient cancellingfield component produced by said gradient cancelling coil means.

1. A superconductive magnet system including, means forming asuperconductive solenoid having a winding with a multitude of turns ofsuperconductor for producing a main magnetic field region within saidsolenoid having certain residual magnetic field gradient components,said winding of said solenoid including a pair of end winding segmentsconnected in series with an intermediate winding segment of more turnsthan each of said end winding segments, each of said winding segmentscomprising a multitude of turns of superconductor, means forming asuperconductor shunt connected in shunt across each of said windingsegments, means including nonsuperconductive current carrying leadsconnected to each end of each of said end winding segments forenergizing said end winding segments from a current supply with currenthaving an amplitude which is separately variable relative to theamplitude of the current in said intermediate winding segment of saidsolenoid to produce a gradient cancelling field component, means forminga switch connected in each of said superconductor shunts which shuntsaid end winding segments, whereby each of said end winding segments maybe energized from the current supply to produce a certain gradientcancelling field component superimposed on the main field of saidsolenoid and then switched via said switch means into a persistentsuperconductive current mode to preserve the preestablished gradientcancelling field component.
 2. The apparatus of claim 1 including, meansforming a cryostat for containing said solenoid means and for coolingsaid solenoid to a superconductive state, means for immersing agyromagnetic resonance sample of matter within the gradient cancelledregion of magnetic field, means for exciting and detecting gyromagneticresonance of the sample.
 3. In a superconductive magnet apparatus, meansforming a superconductive solenoid for producing a main magnetic fieldregion to be corrected which has certain residual magnetic fieldgradient components therein, first and second gradient cancelling coilmeans for cancelling the residual magnetic field gradient components,each of said first and second gradient cancelling coil means having anorthogonal current path geometry relative to each other, asuperconductive joint connected across the ends of each of said firstand second coil means to form first and second closed superconductivecircuits, and wherein said closed superconductive circuits are disposedinside said solenoid surrounding the field region to be corrected,whereby each of said coil means acts as the secondary winding of a DCtransformer and the correct current amplitude will automatically flowthrough each of said orthogonal coil geometries to cancel the certainresidual magnetic field components in the region to be corrected.
 4. Ina superconductive magnet apparatus, means forming a superconductivesolenoid for producing a main magnetic field region to be correctedwhich has certain residual magnetic field gradient components therein,first and second gradient cancelling coil means for cancelling theresidual magnetic field gradient components, each of said first andsecond coil means having an orthogonal current path geometry relative toeach other, means forming a superconductive connection connecting acrossthe ends of each of said first and second coil means to form first andsecond closed superconductive circuits, means forming a direct currenttransformer having a primary winding energized with current proportionalto the current flowing in the windings of said superconductive solenoid,and said superconductive connection which connects across the ends ofeach of said first and second coils being wound to form first and secondsecondary windings of said direct current transformer means, whereby thegradient cancelling field components produced by said first and secondgradient cancelling coil means have magnitudes which vary in proportionto the current flowing in said solenoid, thereby preserving thepreestablished gradient cancelling effect when the main magnetic fieldintensity is scanned.
 5. The apparatus of claim 4 including means forvarying the amount of magnetic coupling between the primary andsecondary windings of said transformer means for adjusting the amplitudeof the gradient cancelling field component produced by said gradientcancelling coil means.